1. Field of the Invention
The present invention generally relates to an optical scanning device for use in an image forming apparatus using an electrophotographic printing process to form an image, and, more particularly, to an optical scanning device in which the multiple light beams, emitted by a plurality of light sources, are deflected by a rotary polygonal mirror and the deflected beams are focused onto a scanned surface of a photosensitive medium so that the photosensitive medium surface is scanned with the light spots. Further, the present invention relates to an image forming apparatus in which the optical scanning device is provided.
2. Description of the Related Art
With the widespread use of image forming systems, such as laser printers and digital copiers, there is an increasing demand for improved print-out speeds of the image forming systems. To meet the requirement, an optical multi-beam scanning device has been proposed for use in the image forming systems. In the multi-beam scanning device, the rotating speed of a rotary polygonal mirror is increased and a plurality of light sources, such as laser diodes, are used to emit multiple light beams for scanning the scanned surface with the multiple light spots at a time.
In the multi-beam scanning device of the above type, the amount of the laser beam emitted by each of the plurality of laser diodes is significantly affected by the magnitude of an electric current flowing through the laser diode, the ambient temperature, and a change in temperature of the laser diode itself. The multi-beam scanning device typically employs a light monitoring sensor to monitor the amount of the laser beam emitted by each of the plurality of laser diodes, and the amount of the laser beam is controlled based on the result of the monitoring by using a feedback circuit.
As disclosed in Japanese Published Application No.7-12709 (corresponding to Japanese Laid-Open Patent Application No.63-163872), an optical scanning device using a semiconductor laser array as the plurality of light sources is known. The amounts of laser beams emitted by the laser diodes of the array are individually detected by a time-division method using a small number of light monitoring sensors. The number of light monitoring sensors is smaller than the number of the light sources. The detection signals indicating the monitored light amounts are produced by the sensors and compared with reference values that are predetermined for the respective light sources. The amounts of the laser beams are controlled based on the results of the monitoring such that the amounts of the laser beams emitted by the laser diodes are set at controlled light amounts.
Similarly, Japanese Laid-Open Patent Application No.7-235715 discloses an optical scanning device using a semiconductor laser array as the plurality of light sources. In the optical scanning device of the above document, one of the light sources is selected and sequentially turned on so that only the on-state light source emits the laser beam. Using a single light monitoring sensor, the amount of the laser beam from the selected light source is monitored. The respective amounts of the laser beams from the plurality of light sources are individually controlled based on the result of the monitoring with respect to the selected light source that is sequentially supplied by the sensor.
Further, as disclosed in Japanese Laid-Open Patent Application No.7-199096, a similar optical scanning device that uses a semiconductor laser chip including two laser diodes is known. The amounts of laser beams emitted by the two laser diodes are individually detected by using a single light monitoring sensor. When one of the two light sources is turned on to emit a laser beam for monitoring, the other light source is set in the off state and does not emit a laser beam. The amounts of the laser beams are controlled based on the result of the monitoring produced by the sensor, such that the amounts of the laser beams emitted by the two light sources are set at a controlled light amount.
In the conventional optical scanning devices of the above documents, the amounts of the laser beams are separately controlled based on the results of the monitoring such that the amounts of the laser beams emitted by the light sources are set at the controlled light amount.
However, the light amount control must be performed during a non-imaging period in which the image formation is not performed. The above documents do not take account of the problem that it takes a relatively long time to monitor and control the light amount of each of the plurality of light sources. The larger the number of the light sources included in the optical scanning device, the longer the time needed for monitoring the amounts of the emitted laser beams and controlling the light amounts of the light sources.
In the conventional optical scanning devices of the above documents, there is a problem in that the timing of light-amount control of the light sources may interfere with the timing of sync signal detection to start the optical scanning of imaging information of the light sources. In such a case, it is difficult to attain the formation of an image with good quality.
In order to overcome the above-described problems, it is an object of the present invention to provide an improved optical scanning device that carries out the light-amount control for a plurality of light sources and safely prevents, with a simple, inexpensive configuration, the undesired interference between the timing of light-amount control of the light sources and the timing of sync signal detection to start the optical scanning of imaging information of the light sources.
Another object of the present invention is to provide an image forming apparatus in which an optical scanning device is provided, the optical scanning device safely preventing, with a simple, inexpensive configuration, the undesired interference between the timing of light-amount control of the light sources and the timing of sync signal detection to start the optical scanning of imaging information of the light sources.
Another object of the present invention is to provide an optical scanning method that carries out the light-amount control for a plurality of light sources and safely prevents, with a simple, inexpensive configuration, the undesired interference between the timing of light-amount control of the light sources and the timing of sync signal detection to start the optical scanning of imaging information of the light sources.
The above-mentioned objects of the present invention are achieved by an optical scanning device which comprises: a plurality of light sources emitting a plurality of light beams; an optical system having a rotary polygonal mirror deflecting the light beams from the light sources, the optical system focusing the deflected light beams onto a scanned surface to form light spots thereon, the scanned surface being optically scanned with the light spots by a rotation of the rotary polygonal mirror; a light-amount monitoring unit monitoring respective amounts of the light beams emitted by the light sources; and a control unit controlling the respective amounts of the light beams emitted by the light sources, based on detection signals output from the light-amount monitoring unit, wherein the optical scanning device is configured to satisfy the formula: A less than Txe2x88x92mxc2x7t where A is an imaging period from a start of the optical scanning of imaging information to an end of the optical scanning, m is the number of the light sources, t is a time needed to complete the control of the light amount of one of the light sources after monitoring of the amount of the light beam emitted by that light source, T is a scanning period indicated by the equation T=60/(Rxc2x7n), R is the number of revolutions per minute of the rotary polygonal mirror, and n is the number of reflection surfaces of the rotary polygonal mirror.
The above-mentioned objects of the present invention are achieved by an image forming apparatus in which an optical scanning device is provided, the optical scanning device comprising: a plurality of light sources emitting a plurality of light beams; an optical system having a rotary polygonal mirror deflecting the light beams from the light sources, the optical system focusing the deflected light beams onto a scanned surface to form light spots thereon, the scanned surface being optically scanned with the light spots by a rotation of the rotary polygonal mirror; a light-amount monitoring unit monitoring respective amounts of the light beams emitted by the light sources; and a control unit controlling the respective amounts of the light beams emitted by the light sources, based on detection signals output from the light-amount monitoring unit, wherein the optical scanning device is configured to satisfy the formula: A less than Txe2x88x92mxc2x7t where A is an imaging period from a start of the optical scanning of imaging information to an end of the optical scanning, m is the number of the light sources, t is a time needed to complete the control of the light amount of one of the light sources after monitoring of the amount of the light beam emitted by that light source, T is a scanning period indicated by the equation T=60/(Rxc2x7n), R is the number of revolutions per minute of the rotary polygonal mirror, and n is the number of reflection surfaces of the rotary polygonal mirror.
The above-mentioned objects of the present invention are achieved by an optical scanning method which comprises the steps of: providing a plurality of light sources emitting a plurality of light beams; providing an optical system having a rotary polygonal mirror deflecting the light beams from the light sources, the optical system focusing the deflected light beams onto a scanned surface to form light spots thereon, the scanned surface being optically scanned with the light spots by a rotation of the rotary polygonal mirror; monitoring respective amounts of the light beams emitted by the light sources; and controlling the respective amounts of the light beams emitted by the light sources, based on results of the light-amount monitoring, wherein the optical scanning method is configured to satisfy the formula: A less than Txe2x88x92mxc2x7t where A is an imaging period from a start of the optical scanning of imaging information to an end of the optical scanning, m is the number of the light sources, t is a time needed to complete the control of the light amount of one of the light sources after monitoring of the amount of the light beam emitted by that light source, T is a scanning period indicated by the equation T=60/(Rxc2x7n), R is the number of revolutions per minute of the rotary polygonal mirror, and n is the number of reflection surfaces of the rotary polygonal mirror.
In the optical scanning device and the image forming apparatus of the present invention, the elements of the optical scanning device are configured to satisfy the predetermined formula that is derived from a proper relationship between the non-imaging period and the timing of the light-amount control of each of the plurality of light sources. It is possible that the optical scanning device and the image forming apparatus of the present invention safely prevent, with a simple, inexpensive configuration, the interference between the timing of the light-amount control of the light sources and the timing of the sync signal detection to start the optical scanning of the imaging information of the light sources.